1. Field of the Invention
This invention relates to a method of operating a heat pump for the purpose of acquiring a high-temperature fluid that is a high quality fluid, such as steam, boiling water, etc. More particularly, this invention provides a method of operating a heat pump characterized by utilizing effectively a subcool region of a condenser.
2. Prior Art
Heat pumps are utilized in a wide variety of applications for heat or cold, for example, refrigeration systems, space cooling or heating systems, hot water heating, etc.
High temperature heat such as heat of steam or boiling water is a high quality energy since storage of such heat is enabled with a high density, an installation (e.g. room heater) for the receipt of heat can be miniaturized, radiant space heating that is silent and moderate is possible, its application range is significantly enlarged because of its sterilizing ability, drying ability, cleaning ability, etc. Consequently, a technology of acquiring heat of such a high temperature efficiently with a heat pump is earnestly expected from many fields.
A major problem with heat pumps is that it is difficult to obtain heat of a high temperature and consequently, how we can attain a highest possible output temperature has been a matter of great concern. Many attempts have been made to that end, but a high temperature on the order of 70.degree.-80.degree. C. at the utmost has been attained.
Attempts to attain such a high temperature include, for example, a method of collecting selectively and efficiently super heat of condensers which are each of a counterflow, single path type (Brit. Patent No. 1 559 318), or a heat pump system comprising counterflow type multiple condensers operating at different multiple pressure levels and multiple expansion means (WO 83/04088). These known methods are aimed at high temperature of 160.degree.-200.degree. F. (ca. 71.degree.-93.degree. C.), but actually acquired is heat of 180.degree. F.(82.degree. C.) at maximum while cold is rejected.
Thus, it has not been possible, so far, to obtain a high-temperature fluid elevated to 100.degree. C. such as boiling water or steam.
A general heat pump having a single circuit shown in FIG. 1b and its operation will be described with reference to FIG. 5a and FIG. 5b:
In an evaporator 4, refrigerant is evaporated at a definite temperature, extracting heat (from fluid to be cooled). When the evaporation is finished (e-f), dry saturated vapor is sucked and compressed with a compressor 1 and delivered at elevated pressure and temperature into a condenser 2 (f-a). The refrigerant vapor at an inlet of the condenser 2 is in superheated state and when a saturated vapor temperature is reached (a-b), liquefaction and condensation begin. The refrigerant is liquefied and condensed as it is cooled by a fluid to be heated (cooling water) until the refrigerant becomes saturated liquid and the condensation is completed (b-c). The liquid refrigerant is further subcooled (c-d) and passed through an expansion valve 3, and thereafter flows back into the evaporator 4 at lowered pressure and temperature (d-e). Thus, a refrigeration cycle is formed, wherein in the evaporator 4 the fluid to be cooled is changed into cold fluid giving up heat to the refrigerant whereas in the condenser 2 the fluid to be heated is changed into hot fluid extracting heat from the refrigerant. The enthalpy change during the refrigeration cycle is shown in a Mollier chart of FIG. 5b and the heat exchange between the refrigerant and the fluid in the condenser is shown in FIG. 5a.
The heat pump operation is also true with a binary heat pump illustrated in FIG. 1a, which comprises a low-temperature stage circuit for circulation of a refrigerant including a compressor 11, an evaporator 14, an expansion valve 13, a cascade condenser/evaporator 22; and a high-temperature stage circuit for circulation of another refrigerant including a compressor 1, the cascade condenser/evaporator 22, an expansion valve 3 and a condenser 2, both circuits being interconnected in a heat exchangeable manner through the cascade condenser/evaporator 22, whereby a fluid to be heated can be discharged as a hot fluid from the condenser 2 and cold fluid can be discharged from the evaporator 14.
For the high-temperature stage circuit, a higher-boiling-point refrigerant such as 1,1,2-trichloro-1,2,2-trifluoroethane (flon R-113), s-dichlorotetrafluoroethane (flon R-114), trichlorofluoromethane (flon R-11), etc. may be used whereas for the low-temperature stage circuit, a lower-boiling-point refrigerant such as dichlorodifluoromethane (flon R-12), chlorodifluoromethane (flon R-22), etc. may be used.
In this manner, conventional refrigeration systems have been operated so as to ensure a certain amount of subcool degree in order to make the expansion valve operative without impairment, and the subcool degree necessitated to cause the expansion valve to act normally is currently considered to be as low as 3.degree.-5.degree. C. at the utmost. A superheat degree varies depending upon the kind of refrigerant, but usually is larger than a subcool degree.
Most condensers have each had a maximum heat transfer coefficient in the saturated refrigerant region and significantly lower heat transfer coefficients in the superheat and supercool regions, and consequently, no attempt to utilize heat transfer characteristics of supercool region has been made and considered. If it is intended to take advantage of supercool degree, the condenser to be used will be too large in size with the result that not only is its economic merit reduced, but also an increased pressure loss owing to the condenser of large size reduces the coefficient of performance. Of conventional heat exchangers for condensers, those of a shell and tube type, a parallel-flow type, a crossflow type, a circulation-counterflow type, a mixed flow type, etc. have been of no use since they cannot sufficiently cool the refrigerant.
Thus, the utilization of heat transmission characteristics of a supercool region has involved many obstacles and consequently, has never been taken into account or has been deemed impossible.
In view of the prior art problems above, this invention is aimed at providing a method of operating a heat pump with which it is possible to acquire a high-temperature fluid of 100.degree. C. or more which is a high-quality fluid, such as steam (ca. 120.degree.), boiling water (ca. 100.degree.C.), etc. as well as relatively high-temperature water of 70.degree.-100.degree. C. More specifically, a primary object of this invention is to provide a method of operating a heat pump which enables it to discharge a high-temperature output fluid, with a maximal fluid temperature difference between the output and input temperatures being 80.degree.-100.degree. C. To that end, the invention is designed to realize the foregoing object through a single condenser without using a large-size condenser or mutliple condensers.
With a view toward attaining the object, the invention has taken a theoretical approach by newly considering the factor of a temperature effectiveness of refrigerant, which gives a measure of supercool degree, as defined by the formula: ##EQU2##
We have investigated into the possibility of attaining efficiently an optimal high supercool degree that is much higher than ever while making the temperature difference between the saturated refrigerant temperature and inlet temperature of the fluid to be heated as large as possible and into requisites of a condenser that permit such a high supercool degree. As a result, the invention has been accomplished by finding a heat pumping method of utilizing efficiently a supercool region of a condenser, whereby it is possible to discharge a high-quality high-temperature fluid.